The transport of neutral amino acids in mammalian cells is broadly described as occurring through two parallel pathways, one Na[unreadable]+[unreadable]-dependent and the other Na[unreadable]+[unreadable]-independent. The "Na[unreadable]+[unreadable]-gradient hypothesis" proposes that the electrochemical potential gradient for Na[unreadable]+[unreadable] serves as the sole energy source for driving accumulation of amino acids by the Na[unreadable]+[unreadable]-dependent system. The source of energy for uptake of Na[unreadable]+[unreadable]-independent amino acids is not known. This research is based on two hypotheses: (1)\the cell membrane potential plays an essential role in the provision of nutrients necessary for cellular metabolism and (2)\a single transport pathway serves for the cellular uptake of both Na[unreadable]+[unreadable]-dependent and Na[unreadable]+[unreadable]-independent neutral amino acids. We have determined in the Ehrlich ascites tumor cell: (1)\the coupling stoichiometry of Na[unreadable]+[unreadable]-amino acid co-transport is consistent with the transport of one Na[unreadable]+[unreadable]/amino acid; (2)\the Na[unreadable]+[unreadable] electrochemical potential gradient is sufficient to provide the energy necessary for amino acid accumulation; (3)\the cell membrane is repolarized by active Na[unreadable]+[unreadable] extrusion following addition of amino acids; and (4)\H[unreadable]+[unreadable] may substitute for Na[unreadable]+[unreadable] as a co-substrate suggesting that the same transport system serves for Na[unreadable]+[unreadable]-dependent and Na[unreadable]+[unreadable]-independent amino acids. Ongoing studies are directed toward the evaluation of proton gradients as sources of energy for Na[unreadable]+[unreadable]-independent amino acid transport and the interactions of Na[unreadable]+[unreadable]-\and H[unreadable]+[unreadable]-dependent co-transport. We anticipate that this investigation will lead to a more complete understanding of the energetics involved in meeting the cell's nutritional requirements. (A)